Apparatus and associated methods to introduce diversity in a multicarrier communication channel

ABSTRACT

An apparatus and associated methods to improve diversity gain while preserving channel throughput in a multicarrier communication channel are generally presented.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/451,110 filed Feb. 27, 2003, entitled “RATE-ONESPACE FREQUENCY BLOCK CODES WITH MAXIMUM DIVERSITY GAIN FOR MIMO-OFDM”by Shao, et al and commonly owned by the assignee of the presentinvention. The disclosure of which is expressly incorporated herein byreference for all purposes.

TECHNICAL FIELD

[0002] Embodiments of the present invention are generally directed towireless communication systems and, more particularly, to an apparatusand associated methods to introduce diversity in a multicarrier wirelesscommunication system.

BACKGROUND

[0003] A multicarrier communication system such as, e.g., OrthogonalFrequency Division Multiplexing (OFDM), Discrete Multi-tone (DMT) andthe like, is typically characterized by a frequency band associated witha communication channel being divided into a number of smaller sub-bands(subcarriers herein). Communication of information (e.g., data, audio,video, etc.) between stations in a multicarrier communication system isperformed by dividing the informational content into multiple pieces(e.g., symbols), and then transmitting the pieces in parallel via anumber of the separate subcarriers. When the symbol period transmittedthrough a subcarrier is longer than a maximum multipath delay in thechannel, the effect of intersymbol interference between the subcarriersmay be significantly reduced.

[0004] By simultaneously transmitting content through a number ofsubcarriers within the channel, multicarrier communication systems offermuch promise for high-throughput wireless applications such as, e.g.,wireless personal area network, local area network, metropolitan areanetwork, fixed broadband wireless access, and the like. Each of thesenetworking environments present their own challenges and, as such, asystem designed to operate in one environment may not be suitable forother environments.

[0005] In broadband wireless access (BWA) networks (e.g., thosedescribed in the IEEE 802.1 6a standard, referred to below), deep fadesmay occur that can persist over a significant period of time. Further,such wide-area wireless channels encounter significant dispersion due tomultipath propagation that limits the maximum achievable rates. SinceBWA is intended to compete with cable modems and xDSL where the channelis static and non-fading, such system designs must counteract these keychallenges and provide high data-rate access at almost wireline quality.To date, conventional techniques such as BLAST, space-time encoding,etc. fail to provide the diversity gain while sustaining the coding rateas the number of transmit antenna increase past two (2). In this regard,such conventional techniques for providing broadband wireless accesstypically have to trade data rate (or, throughput) for received channelquality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Embodiments of the present invention are illustrated by way ofexample, and not by way of limitation, in the figures of theaccompanying drawings in which like reference numerals refer to similarelements and in which:

[0007]FIG. 1 is a block diagram of an example multicarrier wirelessnetwork incorporating the teachings of the present invention, accordingto one example implementation;

[0008]FIG. 2 is a block diagram of an example transceiver architectureincorporating the teachings of the present invention, according to oneexample implementation;

[0009]FIG. 3 is a flow chart illustrating a method for encoding/decodingcontent, according to one example embodiment of the invention;

[0010]FIG. 4 is a graphical illustration of an example rate-one,space-frequency block code matrix, suitable for use in accordance withembodiments of the present invention;

[0011]FIGS. 5, 6 and 7 provide graphical illustration depicting theperformance advantage of embodiments of the present invention againstconventional channel coding techniques, according to embodiments of theinvention; and

[0012]FIG. 8 is a block diagram of an example article of manufactureincluding content which, when executed by an accessing machine, causesthe machine to implement one or more aspects of embodiment(s) of theinvention.

DETAILED DESCRIPTION

[0013] Embodiments of an apparatus and associated methods to introducediversity into a multicarrier wireless communication channel aregenerally presented. More particularly, according to an exampleembodiment, a diversity agent (DA) is introduced that utilizes aninnovative coding scheme to improve diversity gain in a MIMO-OFDM systemover frequency-selective channels, while also providing improvedspace-multipath diversity without rate loss. As will be developed morefully below, the diversity agent employs an innovative rate-onespace-frequency encoding mechanism that is extensible to any number oftransmit antennas, and does not require that the channel be constantover multiple OFDM symbols.

[0014] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in variousplaces throughout this specification are not necessarily all referringto the same embodiment. Furthermore, the particular features, structuresor characteristics may be combined in any suitable manner in one or moreembodiments.

[0015] Example Network Environment

[0016]FIG. 1 illustrates a block diagram of a wireless communicationenvironment within which the teachings of the present invention may bepracticed. As shown, network 100 depicts two devices 102, 104, eachcomprising one or more wireless transmitter(s) and receiver(s)(cumulatively, a transceiver) 108, 116, baseband and media accesscontrol (MAC) processing capability 112, 114, and memory 110, 118, eachcoupled as shown. As used herein, the devices 102, 104 communicateinformation between one another via a multicarrier wirelesscommunication channel 106 established between the transceiver(s) 108,116 through one or more antenna(e) associated with the devices.According to one embodiment, one of the devices 102 may be coupled toanother network 120, e.g., through one or more of a wireless and/orwireline communication medium. In this regard, embodiments of theinvention may well be implemented by a service provider to provide “lastmile” BWA access to one or more end-users in support of other valueadded services, e.g., voice-over-IP (VoIP), internet access, bill-payingservices, voice mail services, and the like.

[0017] According to one example embodiment, one or more of the device(s)102, 104 may utilize a novel diversity agent that employs an innovativerate-one space frequency encoding mechanism described herein. As usedherein, the diversity agent may work with(in) a multicarrier transmitterto selectively map content (e.g., received from a host device,application, agent, etc.) to one or more antenna(e) and/or OFDM tones togenerate a MIMO-OFDM communication channel 106.

[0018] According to one aspect of the invention, the diversity agent mayinclude a novel rate-one space-frequency (SF) encoder suitable for usewithin a multicarrier communication system with M transmit antenna(e)and N receive antenna(e) to improve the channel diversity offrequency-selective channels. As developed more fully below, therate-one SF code employed by the diversity agent may substantiallyachieve the maximum diversity gain attainable over frequency selectivechannels.

[0019] According to one aspect of the invention, the SF code symbol mayonly consume one multicarrier communication channel block duration and,as such, has a smaller processing delay than conventional multicarrierencoding techniques. Thus, for example, the SF code symbol may last formerely one OFDM block time. In this regard, the rate-one space-frequencyencoder employed by diversity agent may well maximize channel diversitywhile preserving channel throughput in a frequency-selectivemulticarrier channel, making it well suited for the BWA networkenvironment.

[0020] In addition to the foregoing, the diversity agent may selectivelyimplement an innovative technique(s) for decoding information from areceived OFDM channel processed as above, although the scope of theinvention is not limited in this regard. Accordingly, a receivediversity agent is introduced which may include one or more of acombiner and a decoder to decode at least a subset of received signalelements encoded with the code construction described herein. Accordingto one embodiment, the diversity agent may employ a maximum ratiocombiner to receive a number of signal(s) and generate a signal vector.Diversity agent may also include a sphere decoder, coupled with thecombiner, to decode the resultant signal vector received from thecombiner element.

[0021] As used herein, baseband and MAC processing element(s) 112, 114may be implemented in one or more processors (e.g., a baseband processorand an application processor), although the invention is not limited inthis regard. As shown, the processor element(s) 112, 114 may couple tomemory 110, 118, respectively, which may include volatile memory such asDRAM, non-volatile memory such as Flash memory, or alternatively mayinclude other types of storage such as a hard disk drive, although thecope of the invention is not limited in this respect. Some portion orall of memory 110, 118 may well be disposed within the same package asthe processor element(s) 112, 114, or may be disposed on an integratedcircuit or some other medium external to element(s) 112, 114. Accordingto one embodiment, baseband and MAC processing element(s) 112, 114 mayimplement at least a subset of the features of diversity agent describedbelow, and/or may provide control over a diversity agent implementedwithin an associated transceiver (108, 116), although the invention isnot limited in this regard.

[0022] While not specifically denoted in FIG. 1, the diversity agent maywell be implemented in one or more of the baseband and MAC processingelement(s) (112, 114) and/or the transceiver element(s) (108, 116),although the invention is not so limited. As used herein, but for theintroduction of the diversity agent developed more fully below, devices102, 104 are intended to represent any of a wide range of electronicdevices with wireless communication capability including, for example, alaptop, palmtop or desktop computer, a cellular telephone (e.g., a 2 G,2.5 G, 3 G or 4 G handset), a personal digital assistant, an WLAN accesspoint (AP), a WLAN station (STA), and the like.

[0023] According to one embodiment, network 100 may represent abroadband wireless access (BWA) network wherein one or more of device(s)102, 104 may establish a wireless communication channel in accordancewith the specification of the Institute for Electrical and ElectronicsEngineers IEEE Std. 802.16-2001 IEEE Std. 802.16-2001 IEEE Standard forLocal and Metropolitan area networks Part 16: Air Interface for FixedBroadband Wireless Access Systems, and its progeny including, e.g., IEEEStd 802.16a-2003 (Amendment to IEEE Std 802.16-2001), although theinvention is not limited in this regard.

[0024] As used herein, network 120 is intended to represent any of abroad range of communication networks including, for example a plain-oldtelephone system (POTS) communication network, a local area network(LAN), metropolitan area network (MAN), wide-area network (WAN), globalarea network (Internet), cellular network, and the like. According toone example implementation, device 102 represents an access point (AP),while device 104 represents a station (STA), each of which suitable foruse within an IEEE 802.16 wireless network, although the invention isnot limited in this regard.

[0025] Example Architecture(s)

[0026] Turning to FIG. 2, a block diagram of an example transmitterarchitecture and an example receiver architecture are presentedaccording to embodiments of the invention. To illustrate thesearchitectures within the context of a communication channel between twodevices, a transmitter from one device (e.g., 102) and a receiver fromanother device (e.g., 104) associated with a communication link aredepicted. Those skilled in the art will appreciate that a transceiver ineither device (102, 104) may well comprise the transmitter architectureand/or the receiver architecture as detailed in FIG. 2, although thescope of the invention is not limited in this regard. It should beappreciated that transmitter and receiver architectures of greater orlesser complexity that nonetheless implement the innovative transmitdiversity and/or space-frequency interleaving described herein areanticipated by the scope and spirit of the claimed invention.

[0027] According to the example embodiment of FIG. 2, transmitter 200 isdepicted comprising serial-to-parallel converter(s) 202, diversity agent204 incorporating elements of an embodiment of the invention, inversediscrete Fourier transform element(s) 206, cyclical prefix, or guardinterval, insertion element(s) 208, radio frequency (RF) processingelement(s) 210 and two or more antenna(e) 220A . . . M, each coupled asdepicted. According to one embodiment, transmitter architecture 200 maybe implemented within transceiver 108 and/or 116. Although depicted as anumber of separate functional elements, those skilled in the art willappreciate that one or more elements of transmitter architecture 200 maywell be combined into a multi-functional element, and converselyfunctional elements may be split into multiple functional elementswithout deviating from the invention.

[0028] As used herein, serial-to-parallel (S/P) transform 202 mayreceive information (e.g., bits, bytes, frames, symbols, etc.) from ahost device (or, an application executing thereon, e.g., email, audio,video, etc.) for processing and subsequent transmission via thecommunication channel. According to one embodiment, the receivedinformation is in the form of quadrature amplitude modulated (QAM)symbols (i.e., wherein each symbol represents two bits, b_(i) andb_(j)). According to one embodiment, the serial-to-parallel transform202 may generate a number of parallel substreams of symbols, which arepassed to one or more instances of diversity agent 204. Althoughdepicted as a separate functional element, serial to parallel transform202 may well be included within embodiments of diversity agent 204, oranother element of the transmitter 200.

[0029] Diversity agent 204 is depicted comprising one or more of apre-coder 212A . . . Z and a space frequency encoder(s) 214, eachcoupled as depicted according to an example embodiment, although theinvention is not limited in this regard. According to one embodiment,the pre-coder functionality may well be integrated within thespace-frequency encoder 214. In this regard, diversity agent(s) 204 ofgreater or lesser complexity that nonetheless generates aspace-frequency matrix of encoded symbols are anticipated by thedisclosure herein.

[0030] As developed more fully below, diversity agent 204 generates anM×Nc space-frequency matrix, where M is the number of transmitantenna(e) and Nc is the number of subcarriers of the multicarriercommunication channel, using a rate-one space-frequency coding mechanismdescribed more fully below with reference to FIG. 3. It will beappreciated that the rate-one space-frequency code applied to contentreceived by diversity agent 204 may substantially achieve the maximumdiversity attainable over frequency-selective channels. Moreover, sincethe code is transmitted in one OFDM block duration, it has smallerprocessing delay than conventional encoding mechanisms (e.g.,space-time-frequency (STF) block codes).

[0031] According to one embodiment, the space-frequency encoded symbolsof the space frequency may also receive additional processing (e.g.,encoding, mapping, weighting, etc.) within encoder 214. For example,encoder 214 may apply an appropriate modulation technique to thecontent. According to one embodiment, any one or more of BPSK, QPSK,8-PSK, 16 QAM, 64 QAM, 128 QAM, 256 QAM, and the like, modulationtechniques may be used, although the invention is not limited in thisregard. According to one embodiment, in addition to the rate-one spacefrequency coding technique, any of a number of additional encodingtechniques and encoding rates, e.g., ½, ⅔, ¾., ⅚, ⅞, 1, {fraction (4/3)}and the like may well be used. According to one embodiment, a code rateof 1 can be maintained for any number of transmit antennae M and receiveantenna(s) N and, when used in combination with the rate-one spacefrequency encoding techniques described herein, will yield the desireddiversity gain of M N L.

[0032] The space-frequency encoded content is passed from diversityagent 204 to one or more inverse discrete Fourier transform element(s)206, which transforms the content from the frequency domain into thetime domain. According to one embodiment, the IDFT(s) 206 may be inversefast Fourier transform (IFFT) element(s), although the invention is notlimited in this regard. According to one embodiment, the number of IDFTelements 206 may be commensurate with the number of transmit antenna(e)(M), e.g., the number transmit radio frequency (RF) chains.

[0033] The time domain content from the IDFT element(s) 206 may bepassed to CPI element(s) 208, which may introduce a cyclical prefix, ora guard interval in the signal(s), before it is passed to a radiofrequency (RF) front-end 210 for, e.g., amplification and/or filteringprior to subsequent transmission via an associated one or moreantenna(e) 220A . . . M. Thus, an embodiment of multicarriercommunication channel 106 is generated, according to one exampleembodiment of the present invention.

[0034] To extract content processed by a remote transmitter (e.g., 200),an example receiver architecture 250 is introduced. According to oneexample embodiment, receiver 250 is depicted comprising one or more of aradio frequency (RF) front end 254, cyclic prefix (or, guard interval)removal element(s) 256, discrete Fourier transform element(s) 258,receive diversity agent 260 according to embodiments of the invention,and parallel-to-serial transform element(s) 262, each coupled asdepicted to generate a representation (I′) of the originally transmittedinformation.

[0035] As shown, an RF front-end 254 receives a plurality of signalsimpinging on one or more receive antennae 252A . . . N. According to oneembodiment, each receive antenna has a dedicated receive chain, wherethe number of receive front-end elements 254, CPR elements 256 and FFTelements are commensurate with the number (N) of receive antenna(e)(e.g., N).

[0036] The RF front end 254 may pass at least a subset of the receivedsignal(s) to a cyclic prefix removal element(s) 256, although theinvention is not limited in this regard. According to one embodiment,CPR 256 removes any cyclic prefix or guard interval that may have beenintroduced during transmit processing of the received signal(s).

[0037] The content from CPR 256 is then provided to an associated one ormore of discrete Fourier transform (DFT) element(s) 258. According toone embodiment, DFT elements 258 may employ a fast Fourier transform tothe received signals to convert the received signals from a time domainto the frequency domain. Thus, a plurality of frequency domainrepresentations of the received signal(s) are presented to receivediversity agent 260.

[0038] According to one aspect of the present invention, receivediversity agent 260 is presented comprising a combiner element 264coupled to one or more decoder element(s) 266A . . . Z. As developedmore fully in FIG. 3, diversity agent 260 may receive one or more signalvectors at a maximal ratio combiner (264). The maximal ratio combiner264 may phase align the various signal vectors, apply appropriateweighting measures and sum at least a subset of the various vectors. Theoutput vector(s) are then applied to one or more sphere decoder(s) 266A. . . Y.

[0039] As used herein, any of a number of sphere decoder's may well beused as the sphere decoder(s) 266A . . . Y. According to one embodiment,described more fully below, the sphere decoder searches for the closestpoint among lattice points within a sphere of given radius centered atthe receive point. For example, in the case of a QAM constellation, thesphere decoder may traverse the entire lattice (defined by a sphere ofsufficient radius) to identify signal vectors, and then filter out anyvectors that do not belong to the desired constellation, although thescope of the invention is not limited in this regard.

[0040] Once decoded, a number (Y) of parallel substreams of the decodedchannel are provided to parallel-to-serial transform element(s) 262,which generate a serial representation (I′) of the originally processedinformational content (I).

[0041] Example Code Construction and Diversity Agent Operation

[0042] Turning to FIG. 3, a flow chart of an example method of diversityagent operation to improve the diversity gain within a communicationchannel, while maintaining coding rate and channel throughput isgenerally presented, according to embodiments of the invention. For easeof discussion, and not limitation, the brief introduction to the designcriteria and code construction is provided as an introduction to theoperation of the diversity agent.

[0043] Code Design Criteria

[0044] According to one example embodiment, the rate-one,space-frequency code employed herein contemplate use within a MIMO-OFDMsystem with M transmit and N receive antennas and Nc subcarriers, whereNc>>M,N, although the scope of the invention is not limited to suchsystems and is, indeed, extensible to any multicarrier communicationsystem with any number of subcarriers, transmit antenna(e) and receiveantenna(e). Let C and E be two different space-frequency code wordsrepresented by matrices of size M×Nc. Assuming that the MIMO channelconsists of L (matrix) taps, an upper bound on the expected pairwiseerror probability (averaged over the, e.g., general Rayleigh fadingchannel realizations) was derived. For the special case of no spatialfading correlation and a uniform power delay profile, the upper boundcan be expressed as: $\begin{matrix}{{P\left( C\rightarrow E \right)} \leq {\prod\limits_{i = 0}^{{{rank}{(S)}} - 1}\quad \left( {1 + {{\lambda_{i}(S)}\frac{\rho}{4}}} \right)^{- N}}} & (1)\end{matrix}$

[0045] where ρ is the average signal-to-noise ratio (SNR), λ_(i)(S) isthe i-th nonzero eigenvalue of S. S=G(C,E)G^(H)(C,E) has dimension Nc×Ncwhere G(C,E) is the Nc×ML matrix G(C,E)=[(C−E)^(T)D(C−E)^(T) . . .D^(L−1)(C−E)^(T)] and$D = {\left\{ ^{{- j}\frac{2\pi}{Nc}k} \right\}_{k = 0}^{{Nc} - 1}.}$

[0046] For Nc>ML, in order to achieve M N L-fold diversity, appropriatecode design is necessary to ensure that not only the M×Nc error matrix(C−E) is of full rank over all distinct {C,E} pairs, but the stackedmatrix G(C,E) enjoys full rank as well. Just such a code design isintroduced below in the rate-one, space-frequency encoder employed bythe diversity agent.

[0047] According to one embodiment, for rate-one space-frequency blockcodes, the number of information symbols mapped into the space-frequencycode matrix may be equal to the number of subcarriers Nc. According toone example embodiment, Nc=M×L×G, where G is the number of groups thesubcarriers are divided into (i.e., the number of groups of subcarriers)(see, e.g., FIG. 4).

[0048] Rate-One, Space-Frequency Encoding

[0049] With that introduction to the development of the rate-onespace-frequency block code, we turn to FIG. 3 where the method of spacefrequency encoding 300 begins with block 302, wherein diversity agent204 receives input (e.g., from a host device or application executingthereon), block 302. According to one embodiment, the diversity agentreceives QAM symbols, although the invention is not limited in thisregard.

[0050] According to one embodiment, the content is received by one ormore pre-coders 212A . . . Z of diversity agent 204, which may begin theencoding process by dividing the received content into a number (G) ofgroups, block 304. According to one embodiment, the Nc×1 vector of inputsymbols s=[S₀ ^(T) S₁ ^(T) . . . S_(G−1) ^(T)]^(T) is divided into Ggroups of size ML×1 vectors {s_(G)}_(g=0) ^(G−1) although the inventionis not limited in this regard.

[0051] In block 306, at least a subset of the vector of input symbols,S_(g) is multiplied by a constellation-rotation (CR) pre-coder (e.g.,within pre-coder 212) Θ. According to one embodiment, the sameconstellation-rotation Θ is applied to each of the Nc×1 vector of inputsymbols s_(G) by left-multiplying the vector by the constellationrotation, although the invention is not limited in this regard.According to one embodiment, the constellation rotation Θ is ofdimension ML×ML to produce size ML-vector v_(g)=Θs_(g)=[Θ₁ ^(T)s_(g), .. . , Θ_(ML) ^(T)s_(g))]^(T), where Θ_(i) ^(T) denotes the ith row of Θ.

[0052] In block 308, at least a subset of the vectors v_(g) is dividedinto L, M×1 subvectors, which are used to generate an M×M diagonalmatrix D_(s) _(g) _(,k)=diag{Θ_(M×(K−1)+1) ^(T)s_(g), . . . , Θ_(M×K)^(T)s_(g)} for k=1 . . .L. According to one example embodiment, Lsubmatrices are regarded to be in the same group.

[0053] In block 310, the submatrices from the G groups (e.g., a total ofG×L diagonal matrices) are interleaved to generate the M×Ncspace-frequency matrix:

C=[D _(s) ₀ _(,1) , . . . , D _(s) _(G−1) ₁ , . . . , D _(s) ₀ _(, L) ,. . . , D _(s) _(G−1) _(,L)]  (2)

[0054] as depicted in FIG. 4, where {f_(i)}_(i=0) ^(Nc−1) denote the Ncsubcarriers. As a result, submatrices corresponding to successivesymbols in the same group are equi-spaced in the codeword, C.

[0055] Those skilled in the art will appreciate that the rate-one,space-frequency designed described above possesses the property thatsuccessive symbols from the same group transmitted from the same antennaare at a frequency “distance” that is multiples of MG subcarrierspacings in order to exploit the diversity in frequency-selectivechannels. Accordingly, for known channel order and a uniform power delayprofile, the channel frequency responses on subcarriers spaced atmultiples of MG are uncorrelated. Therefore, the L symbols from the samegroup transmitted from the same antenna experience uncorrelated fading.

[0056] According to one embodiment, the rate-one, space-frequencyencoder described above leverages the following desirable property of Θ:for all distinct pairs {s_(g), {tilde over (s)}_(g)} and v_(g)=Θs_(g),and {tilde over (v)}_(g)=Θ{tilde over (s)}_(g), the corresponding errorvector e_(g)=(v_(g)−{tilde over (v)}_(g)) has substantially all nonzeroelements. As a result, generating D_({tilde over (s)}) _(g) _(,k) fork=1 . . . L from {tilde over (s)}_(g), then the L diagonal matrices(D_(s) _(g) _(,k)−D_({tilde over (s)}) _(g) _(,k)) have all diagonalelements that are nonzero. Accordingly, all distinct pairs {s_(g),{tilde over (s)}_(g)} give rise to L full-rank diagonal error matrices,which may be used to prove that the space-frequency codes proposedherein can achieve the maximum diversity gain of M N L. This proof isprovided below.

[0057] Returning to FIG. 3, attention is directed to an example method350 (on FIG. 3) of decoding content encoded using the rate-one,space-frequency encoding technique introduced above. According to oneexample embodiment, the decoding technique employed by diversity agent260 begins with block 352 wherein diversity agent receives a number ofsignal vectors via the (N) receive processing chains. For ease ofexplanation, let r^(j), w^(j) denote the size Nc×1 received signalvector and noise vector at the jth receive antenna, respectively.According to one embodiment, r^(j) is divided into LG size M×1subvectors {r^(j,k,g)} for g, k where the definition of g and k remainconsistent with that introduced for the encoder, above. Similarly, w^(j)can be divided into LG size M×1 subvectors {w^(j,k,g)}.

[0058] As above, Θ_(i) ^(T) denotes the ith row of the constellationrotation matrix Θ, and H_(i,l) ^(j) denotes the channel frequencyresponse of the ith transmit and the jth receive antenna pair at the lthtone. According to one embodiment, the diagonal matrix may be definedas: $\begin{matrix}{\Lambda_{k,g}^{j} = {{diag}\begin{Bmatrix}H_{1,{{{({k - 1})}{GM}} + {gM}}}^{j} \\H_{2,{{{({k - 1})}{GM}} + {gM} + 1}}^{j} \\\vdots \\H_{M,{{{({k - 1})}{GM}} + {{({g + 1})}\quad M} + 1}}^{j}\end{Bmatrix}}} & (3)\end{matrix}$

[0059] Accordingly, for the rate-one, space-frequency coding mechanismdefined above, the received signal vector may be represented as:$\begin{matrix}\begin{matrix}{r^{j} = \left\lbrack {r^{j,1,0}\quad r^{j,1,1}\quad \ldots \quad r^{j,L,{G - 2}}\quad r^{j,L,{G - 1}}} \right\rbrack^{T}} \\{= {\left\lbrack {b^{j,1,0}\quad b^{j,1,1}\quad \ldots \quad b^{j,L,{G - 2}}\quad b^{j,L,{G - 1}}} \right\rbrack^{T} +}} \\{\left\lbrack {w^{j,1,0}\quad w^{j,1,1}\quad \ldots \quad w^{j,L,{G - 2}}\quad w^{j,L,{G - 1}}} \right\rbrack^{T}}\end{matrix} & (4)\end{matrix}$

[0060] where $\begin{matrix}{b^{j,k,g} = {{\Lambda_{k,g}^{j}\begin{bmatrix}\Theta_{{{({k - 1})}M} + 1}^{T} \\\Theta_{{{({k - 1})}M} + 2}^{T} \\\vdots \\\Theta_{kM}^{T}\end{bmatrix}}s_{g}}} & (5)\end{matrix}$

[0061] and r^(j,k,g)=b^(j,k,g)+w^(j,k,g).

[0062] In block 356, diversity agent combines appropriate sub-blocks ofthe subvectors. According to one embodiment, combining the sub-blocks ofthe gth group, we get: $\begin{matrix}\begin{matrix}{r^{j,g} = \left\lbrack {r^{j,1,g}\quad r^{j,2,g}\quad \ldots \quad r^{j,L,g}} \right\rbrack^{T}} \\{{{\underset{\underset{\Lambda_{g}^{j}}{}}{\begin{bmatrix}\Lambda_{1,g}^{j} & \quad & \quad \\\quad & ⋰ & \quad \\\quad & \quad & \Lambda_{L,g}^{j}\end{bmatrix}}\Theta \quad s_{g}} + w^{j,g}}}\end{matrix} & (6)\end{matrix}$

[0063] According to one embodiment, combining the information from thegth group over the N receive antennas is performed using a maximal ratiocombiner element(s) 264, which yields: $\begin{matrix}{y_{g} = {\left( \underset{\underset{\sum_{g}}{}}{\left( {\sum{\left( \Lambda_{g}^{j} \right)^{H}\Lambda_{g}^{j}}} \right)} \right)^{{- 1}/2} \times \left\lbrack {\left( \Lambda_{g}^{1} \right)^{H}\ldots \quad \left( \Lambda_{g}^{N} \right)^{H}} \right\rbrack r_{g}}} & (7) \\{{= {{\sum_{g}^{1/2}{\Theta \quad s_{g}}} + \eta_{g}}}\quad} & (8)\end{matrix}$

[0064] In block 358, diversity agent may use a maximal likelihood (ML)decoding technique of order 2×L×M to decode s_(g) from y_(g), althoughthe invention is not limited in this regard. According to one exampleimplementation, introduced above, one or more sphere decoder element(s)266A . . . Y may well be employed in this regard.

[0065] Accordingly, example encoding and decoding techniques toimplement the rate-one, space-frequency code introduced herein have beendescribed. It can be shown by the proof (provided below) that therate-one space-frequency block codes proposed herein can achieve themaximum diversity gain of M N L.

[0066] Before delving into the supporting proof, attention is nowdirected to FIG. 4, which provides a graphical representation of thestructure of a rate-one, space-frequency code matrix C, according to butone example embodiment of the invention. With reference to FIG. 4,matrix 400 depicts the general case of M antennae, G subcarrier groups,and L matrix channel taps. Matrix 420 depicts an example implementationwith four (4) transmit antenna (M), 16 subcarriers (L) split into twogroups. As shown, the notation in the blocks (s_i,j,k) represent the kthprecoded symbol (s) in the jth group of the ith layer.

[0067] Turning to FIGS. 5-7, graphical representations of variousperformance comparison's are provided, according to one exampleembodiment, each of which will be addressed in turn. According to oneembodiment, for purposes of these simulations, an OFDM system conformingto the 802.16.3 standard was used with FFT size of 256. Modulationsymbols used were BPSK, 4 QAM (or, 16 QAM) where the total averagesymbol energy on M transmit antennas E_(s)=1.

[0068] In FIG. 5, a graphical illustration of throughput quality (e.g.,quantified as a bit error rate (BER)) mapped against an signal-to-noiseratio (Es/No) for a number of system models with 2 transmit antenna, 1receive antenna and different channel taps (L=2, 3, 4). In particular,FIG. 5 depicts that the more channel taps, the better the BERperformance as the Es/No figure increases.

[0069] In FIG. 6, a graphical illustration of an example performancecomparison between the proposed rate-one, space-frequency encodingtechnique and a number of conventional techniques. Of particularinterest is that the space-frequency coding scheme introduced hereinprovides the best performance, and the only one that achieves M N Ldiversity gain.

[0070] In FIG. 7, a graphical illustration of an example performancecomparison between the space-frequency block code and anotherconventional technique (space-time-frequency coding). As shown, therate-one space-frequency coding technique described herein performs onpar with the STF coding technique, while enjoying reduced complexity inthe transmitter and the receiver.

[0071] Proof: Rate-One, Space-Frequency Coding Mechanism Provides M N LDiversity Gain

[0072] From the equation (1) defining the upper bound on the expectedpairwise error probability for the BWA communication environment,proving the diversity gain is substantially equivalent to proving thatrank(S)=ML. Since S=G(C,E)G^(H)(C,E) and rank(S)=rank(G(C,E)) isequivalent to rank (G(C,E)^(T)), it suffices to show thatrank(G(C,E)^(T))=ML, where G(C,E)^(T) is a ML×Nc matrix: $\begin{matrix}{{G\left( {C,E} \right)}^{T} = {\begin{matrix}\left( {C - E} \right) \\{\left( {C - E} \right)D} \\\vdots \\{\left( {C - E} \right)D^{L - 1}}\end{matrix}}} & (9)\end{matrix}$

[0073] and$D = {{diag}{\left\{ ^{{- j}\frac{2\pi}{Nc}k} \right\}_{k = 0}^{{Nc} - 1}.}}$

[0074] For the proof, it is assumed that the number of taps (L) isknown.

[0075] Consider the Nc×1 vectors s=[s₀ ^(T) s₁ ^(T) . . . s_(G−1)^(T)]^(T), and {tilde over (s)}=[{tilde over (s)}₀ ^(T) {tilde over(s)}₁ ^(T) . . . {tilde over (s)}_(G−1) ^(T)]^(T) such that s≠{tildeover (s)} for some g that is an element of {0, . . . , G−1}. Withoutloss of generality, let s₀≠{tilde over (s)}₀.

[0076] Define the diagonal M×M matrix A_((k−1)G+g+1)=D_(s) _(g)_(,k)−D_({tilde over (s)}) _(g) _(,k) for k=1, . . . , L and g=0, . . ., G−1.

[0077] Thus, C−E=[A₁ . . . A_(GL)]. On the other hand, we can divide thediagonal matrix$D = {{diag}\left\{ ^{{- j}\frac{2\pi}{Nc}k} \right\}_{k = 0}^{GL}}$

[0078] into GL M×M diagonal sub-matrices {D_(i)}_(i=1) ^(GL).

[0079] As a result, it can be shown: $\begin{matrix}{{\left( {C - E} \right)D^{i}} = {\quad{{\left\lbrack {A_{1}\quad \ldots \quad A_{GL}} \right\rbrack \begin{bmatrix}D_{1}^{i} & 0 & \ldots \\0 & ⋰ & 0 \\0 & \ldots & D_{GL}^{i}\end{bmatrix}} = {\left\lbrack {D_{1}^{i}\quad \ldots \quad D_{GL}^{i}} \right\rbrack \begin{bmatrix}A_{1} & 0 & \ldots \\0 & ⋰ & 0 \\0 & \ldots & A_{GL}\end{bmatrix}}}}} & (10)\end{matrix}$

[0080] since both Aj and D^(i) _(j) are diagonal matrices. Therefore wecan show: $\begin{matrix}{{G\left( {C,E} \right)}^{T} = {\begin{bmatrix}I_{M} & I_{M} & \ldots & I_{M} \\D_{1} & D_{2} & \ldots & D_{GL} \\\vdots & \vdots & \vdots & \vdots \\D_{1}^{L - 1} & D_{2}^{L - 1} & \ldots & D_{GL}^{L - 1}\end{bmatrix} \times \begin{bmatrix}A_{1} & 0 & \ldots \\0 & ⋰ & 0 \\0 & \ldots & A_{GL}\end{bmatrix}}} & (11)\end{matrix}$

[0081] As previously shown, over all distinct pairs {s₀, {tilde over(s)}₀}, we have L full rank diagonal error matrices A_((k−1)G+1)=(D_(s)₀ _(,k)−D_({tilde over (s)}) ₀ _(,k)) for k=1, . . . L. Thus, inG(C,E)^(T), we can find a ML×ML submatrix that is the product of twoother ML×ML matrices, as follows:

[0082] Accordingly, it may be shown that the product of the full rankblock Vandermonde and full rank block diagonal matrix above has nonzerodeterminant and thus is of full rank ML. Since in G(C,E)^(T) of ML×Nc,we can find a submatrix of dimension ML×ML which is of full rank, weconclude that rank(S)=rank(G(C,E)^(T))=ML is ensured, and thespace-frequency codes proposed herein can achieve the maximum diversitygain of M N L.

ALTERNATE EMBODIMENT(S)

[0083]FIG. 8 illustrates a block diagram of an example storage mediumcomprising content which, when invoked, may cause an accessing machineto implement one or more aspects of the diversity agent 204, 260 and/orassociated methods 300. In this regard, storage medium 800 includescontent 802 (e.g., instructions, data, or any combination thereof)which, when executed, causes an accessing appliance to implement one ormore aspects of the diversity agent 204, 260 described above.

[0084] The machine-readable (storage) medium 800 may include, but is notlimited to, floppy diskettes, optical disks, CD-ROMs, andmagneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or opticalcards, flash memory, or other type of media/machine-readable mediumsuitable for storing electronic instructions. Moreover, the presentinvention may also be downloaded as a computer program product, whereinthe program may be transferred from a remote computer to a requestingcomputer by way of data signals embodied in a carrier wave or otherpropagation medium via a communication link (e.g., a modem, radio ornetwork connection). As used herein, all of such media is broadlyconsidered storage media.

[0085] It should be understood that embodiments of the present inventionmay be used in a variety of applications. Although the present inventionis not limited in this respect, the circuits disclosed herein may beused in many apparatuses such as in the transmitters and receivers of aradio system. Radio systems intended to be included within the scope ofthe present invention include, by way of example only, wireless localarea networks (WLAN) devices and wireless wide area network (WWAN)devices including wireless network interface devices and networkinterface cards (NICs), base stations, access points (APs), gateways,bridges, hubs, cellular radiotelephone communication systems, satellitecommunication systems, two-way radio communication systems, one-waypagers, two-way pagers, personal communication systems (PCS), personalcomputers (PCs), personal digital assistants (PDAs), sensor networks,personal area networks (PANs) and the like, although the scope of theinvention is not limited in this respect.

[0086] The types of wireless communication systems intended to be withinthe scope of the present invention include, although not limited to,Wireless Local Area Network (WLAN), Wireless Wide Area Network (WWAN),Code Division Multiple Access (CDMA) cellular radiotelephonecommunication systems, Global System for Mobile Communications (GSM)cellular radiotelephone systems, North American Digital Cellular (NADC)cellular radiotelephone systems, Time Division Multiple Access (TDMA)systems, Extended-TDMA (E-TDMA) cellular radiotelephone systems, thirdgeneration (3G) systems like Wide-band CDMA (WCDMA), CDMA-2000, and thelike, although the scope of the invention is not limited in thisrespect.

[0087] Embodiments of the present invention may also be included inintegrated circuit blocks referred to as core memory, cache memory, orother types of memory that store electronic instructions to be executedby the microprocessor or store data that may be used in arithmeticoperations. In general, an embodiment using multistage domino logic inaccordance with the claimed subject matter may provide a benefit tomicroprocessors, and in particular, may be incorporated into an addressdecoder for a memory device. Note that the embodiments may be integratedinto radio systems or hand-held portable devices, especially whendevices depend on reduced power consumption. Thus, laptop computers,cellular radiotelephone communication systems, two-way radiocommunication systems, one-way pagers, two-way pagers, personalcommunication systems (PCS), personal digital assistants (PDA's),cameras and other products are intended to be included within the scopeof the present invention.

[0088] The present invention includes various operations. The operationsof the present invention may be performed by hardware components, suchas those shown in FIGS. 1 and/or 2, or may be embodied inmachine-executable content (e.g., instructions) 802, which may be usedto cause a general-purpose or special-purpose processor or logiccircuits programmed with the instructions to perform the operations.Alternatively, the operations may be performed by a combination ofhardware and software. Moreover, although the invention has beendescribed in the context of a computing appliance, those skilled in theart will appreciate that such functionality may well be embodied in anyof number of alternate embodiments such as, for example, integratedwithin a communication appliance (e.g., a cellular telephone).

[0089] In the description above, for the purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout some of these specific details. In other instances, well-knownstructures and devices are shown in block diagram form. Any number ofvariations of the inventive concept are anticipated within the scope andspirit of the present invention. In this regard, the particularillustrated example embodiments are not provided to limit the inventionbut merely to illustrate it. Thus, the scope of the present invention isnot to be determined by the specific examples provided above but only bythe plain language of the following claims.

[0090] What is claimed is:

1. A method comprising: receiving content for transmission via amulticarrier wireless communication channel; and generating a rate-one,space-frequency code matrix from the received content for transmissionon the multicarrier wireless communication channel from a plurality oftransmit antennae.
 2. A method according to claim 1, wherein thereceived content is a vector of input symbols (s) of size Nc×1, whereinNc is the number of subcarriers of the multicarrier wirelesscommunication channel.
 3. A method according to claim 2, the element ofgenerating a rate-one space frequency code matrix comprising: dividingthe vector of input symbols into a number G of groups to generatesubgroups; and multiplying at least a subset of the subgroups by aconstellation rotation precoder to produce a number G of pre-codedvectors (v_(g)).
 4. A method according to claim 3, further comprising:dividing each of the pre-coded vectors into a number of LM×1 subvectors;and creating an M×M diagonal matrix D_(s) _(g) _(,k)=diag{Θ^(T)_(M×(k−1)+1)s_(g), . . . , Θ^(T) _(M×k)s_(g)}, where k=1 . . . L fromthe subvectors.
 5. A method according to claim 4, further comprising:interleaving the L submatrices from the G groups to generate an M×Ncspace-frequency matrix.
 6. A method according to claim 5, wherein thespace-frequency matrix provides M N L channel diversity, whilepreserving a code rate of 1 for any number of transmit antenna(s) M,receive antenna(s) N and channel tap(s) L.
 7. A method according toclaim 1, wherein the space-frequency matrix provides M N L channeldiversity, while preserving a code rate of 1 for any number of transmitantenna(s) M, receive antenna(s) N and channel tap(s) L.
 8. A storagemedium comprising content which, when executed by an accessingcommunications device causes the communications device to implement amethod according to claim
 1. 9. An apparatus comprising: a diversityagent to receive content for transmission via a multicarrier wirelesscommunication channel, and to generate a rate-one, space-frequency codematrix from the received content for transmission on the multicarrierwireless communication channel from a plurality of transmit antennae.10. An apparatus according to claim 9, wherein the received content is avector of input symbols (s) of size Nc×1, wherein Nc is the number ofsubcarriers of the multicarrier wireless communication channel.
 11. Anapparatus according to claim 10, the diversity agent further comprising:a pre-coder element, to divide the vector of input symbols into a numberG of groups to generate subgroups, and to multiply at least a subset ofthe subgroups by a constellation rotation pre-coder to produce a numberG of pre-coded vectors (v_(g)).
 12. An apparatus according to claim 11,the diversity agent further comprising: a space-frequency encodingelement, responsive to the pre-coder element, to divide each of thepre-coded vectors into a number of LM×1 subvectors, and to create an M×Mdiagonal matrix D_(s) _(g) _(,k)=diag{Θ^(T) _(M×(k−1)+1)s_(g), . . . ,Θ^(T) _(M×k)s_(g)}, where k=1 . . . L from the subvectors.
 13. Anapparatus according to claim 12, wherein the space-frequency encodingelement interleaves the L submatrices from the G groups to generate anM×Nc space-frequency matrix.
 14. An apparatus according to claim 13,wherein the space-frequency matrix provides M N L channel diversity,while preserving a code rate of 1 for any number of transmit antenna(s)M, receive antenna(s) N and channel tap(s) L.
 15. An apparatus accordingto claim 9, wherein the space-frequency matrix provides M N L channeldiversity, while preserving a code rate of 1 for any number of transmitantenna(s) M, receive antenna(s) N and channel tap(s) L.
 16. A systemcomprising: a number M of omnidirectional antennas; and a diversityagent, to receive content for transmission via a multicarrier wirelesscommunication channel, and to generate a rate-one, space-frequency codematrix from the received content for transmission on the multicarrierwireless communication channel from at least a subset of the Momnidirectional antennas.
 17. A system according to claim 16, whereinthe received content is a vector of input symbols (s) of size Nc×1,wherein Nc is the number of subcarriers of the multicarrier wirelesscommunication channel.
 18. A system according to claim 17, the diversityagent further comprising: a pre-coder element, to divide the vector ofinput symbols into a number G of groups to generate subgroups, and tomultiply at least a subset of the subgroups by a constellation rotationpre-coder to produce a number G of pre-coded vectors (v_(g)).
 19. Asystem according to claim 18, the diversity agent further comprising: aspace-frequency encoding element, responsive to the pre-coder element,to divide each of the pre-coded vectors into a number of LM×1subvectors, and to create an M×M diagonal matrix D_(s) _(g)_(,k)=diag{Θ^(T) _(M×(k−1)+1)s_(g), . . . , Θ^(T) _(M×k)s_(g)}, wherek=1 . . . L from the subvectors.
 20. A system according to claim 19,wherein the space-frequency encoding element interleaves the Lsubmatrices from the G groups to generate an M×Nc space-frequencymatrix.
 21. A system according to claim 20, wherein the space-frequencymatrix provides M N L channel diversity, while preserving a code rate of1 for any number of transmit antenna(s) M, receive antenna(s) N andchannel tap(s) L.
 22. A system according to claim 16, wherein thespace-frequency matrix provides M N L channel diversity, whilepreserving a code rate of 1 for any number of transmit antenna(s) M,receive antenna(s) N and channel tap(s) L.